In-Depth Understanding of Magnetic Hysteresis Loss in Magnetic Materials: Phenomenon, Principle and Application
Magnetic Hysteresis Loss
1. Definition of Magnetic Hysteresis Loss
Magnetic hysteresis loss, with its English name being “magnetic hysteresis loss”, refers to the energy consumed due to the hysteresis phenomenon when ferromagnetic materials undergo repeated magnetization. When a ferromagnetic substance is in an alternating magnetic field, the change in its internal magnetic induction intensity always lags behind that of the magnetic field intensity. This “magnetic lag” phenomenon causes energy consumption in each magnetization cycle, and this part of energy loss is exactly magnetic hysteresis loss. Figuratively speaking, it is similar to pushing an object with inertia—extra effort is needed every time we change its direction of motion. In ferromagnetic materials, when the direction of the magnetic field intensity changes, the rearrangement of magnetic domains has to overcome a certain resistance, which leads to energy consumption.
2. Generation Mechanism of Magnetic Hysteresis Loss
A. Magnetic Domain Structure and Magnetization Process
The microstructure of magnetic materials consists of numerous magnetic domains. In the unmagnetized state, the magnetic moment directions of the magnetic domains are disorganized, so the material shows no magnetism externally. When an external magnetic field is applied, the magnetic moments of the magnetic domains will gradually align in the same direction, and this process is called magnetization. During magnetization, the domain walls will displace, and the directions of the magnetic domain moments will also rotate. However, when the magnetic field decreases to zero, the magnetic domain structure does not fully return to the state before the magnetic field was applied, and the magnetic induction intensity is generally non-zero. This reflects the irreversibility of the magnetization process, which is one of the root causes of magnetic hysteresis loss. For example, we can imagine magnetic domains as small magnetic needles—without an external magnetic field, they point randomly in various directions. When there is an external magnetic field, the small needles start to align along the direction of the magnetic field. But when the magnetic field disappears, some of the small needles do not fully return to their original disorganized state and retain a certain orientation, which results in energy loss.
B. The Mystery of Hysteresis Loop
The change in the magnetization state of ferromagnetic materials can be described by a hysteresis loop. Starting from the origin, as the magnetic field intensity increases from zero, the magnetic induction intensity rises accordingly. When the magnetic field intensity increases to a certain value, the magnetic induction intensity reaches a saturation value. After that, when the magnetic field intensity decreases, the magnetic induction intensity does not return along the original path but changes with a lag. When the magnetic field intensity drops to zero, the magnetic induction intensity is non-zero, and this value is called the residual magnetization intensity. To reduce the magnetic induction intensity to zero, a reverse magnetic field intensity needs to be applied, and the value of this reverse magnetic field intensity is called the coercive force. The area enclosed by the hysteresis loop represents the magnetic hysteresis loss generated per unit volume of the ferromagnetic material during one cycle of magnetization by the alternating magnetic field. Simply put, the hysteresis loop is like a “trajectory map” of the magnetization process of ferromagnetic materials—the larger the area, the more energy is consumed during magnetization and demagnetization, that is, the greater the magnetic hysteresis loss.
3. Calculation Methods of Magnetic Hysteresis Loss
A. Theoretical Calculation
In the quasi-static repeated magnetization process, the magnetic hysteresis loss per unit volume of the ferromagnetic material generated during one cycle of magnetization by the alternating magnetic field is proportional to the area enclosed by the hysteresis loop. Let the frequency of the alternating magnetic field be f , then the magnetic hysteresis loss P_h per unit time and unit volume is P_h = f \times \text{Area of Hysteresis Loop} . However, since the shapes of the hysteresis loops of various transformer cores are different, and the area of the hysteresis loop is related to the magnetic flux density increment, magnetic permeability, operating frequency, etc., it is relatively difficult to accurately calculate the area of the hysteresis loop. For example, for hysteresis loops with complex shapes, it is hard to accurately calculate their areas using conventional mathematical methods.
B. Empirical Formulas
To facilitate the calculation of magnetic hysteresis loss, people have summarized some empirical formulas. Rayleigh’s Law: In the case of a weak magnetic field (Rayleigh region), the magnetic hysteresis loss is proportional to the square of the magnetic field intensity; in the case of a slightly higher magnetic field, it is proportional to the cube of the magnetic field intensity. Nevertheless, this law only applies to specific magnetic field ranges and material properties. The power of magnetic hysteresis loss is expressed as P_h = fW_h = \frac{4}{3}bH_m^3f .
4. Influencing Factors of Magnetic Hysteresis Loss
A. Material Type
Different types of magnetic materials have completely different hysteresis loops, which directly determine the magnitude of magnetic hysteresis loss. Generally speaking, soft magnetic materials have high magnetic permeability and low coercive force, so their hysteresis loops have small areas and the magnetic hysteresis loss is also low. For instance, silicon steel sheets are usually used as the material for transformer cores precisely because of their low magnetic hysteresis loss, which can effectively improve the efficiency of transformers.
B. Temperature
Temperature has a significant impact on the magnetic hysteresis loss of magnetic materials. As the temperature rises, the magnetization process of magnetic materials becomes relatively easier, and the magnetic hysteresis loss will decrease. However, when the temperature exceeds the Curie temperature of the material, the magnetic material will lose its magnetism, and the magnetic hysteresis loss will increase sharply. For example, some magnetic materials have small magnetic hysteresis loss at room temperature, but when the temperature rises close to the Curie temperature, their magnetic hysteresis loss increases significantly, leading to a sharp decline in equipment performance.
C. Magnetic Field Frequency
The influence of magnetic field frequency on magnetic hysteresis loss also cannot be ignored. When the magnetic field frequency is low, the magnetic domains have sufficient time for magnetization and demagnetization, so the magnetic hysteresis loss is relatively small. When the magnetic field frequency is high, the magnetization and demagnetization processes of the magnetic domains are restricted, and the magnetic hysteresis loss will increase.
In electronic transformers, magnetic hysteresis loss is an important component of core loss. It causes the magnetic core to generate heat, which not only reduces the efficiency of the transformer but also may affect its service life.
As an important performance index of ferromagnetic materials when applied in alternating magnetic fields, magnetic hysteresis loss profoundly affects the performance and efficiency of many electrical devices. By deeply understanding the definition, generation mechanism, calculation methods, influencing factors of magnetic hysteresis loss, as well as the coping strategies in different fields, we can better design and optimize electrical equipment, improve energy utilization efficiency, and promote the continuous development of power engineering and electronic technology.